Chair base



FIG. 1 is a view in front elevation of a chair base showing my new design;

FIG. 2 is a view in rear elevation thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof; and,

FIG. 5 is a view in right side elevation thereof with the view in left side elevation being a mirror image of the view in right side elevation.

The broken lines shown in FIGS. 3 and 4 are for the purpose of illustrating portions of the article and form no part of the claimed design. 

CLAIM The ornamental design for a chair base, as shown and described. 